While JSON has a benefit in terms of size (and so bandwidth) and complexity (slightly easier to parse for smartphones), such major change (moving from HTML to JSON) would be too disruptive to justify the minor gains in bandwidth and performance. Such gains were maybe relevant ten years ago where mobile devices were very limited in terms of CPU and when many people had only access to low-speed internet, but today, it's much less relevant.

Note that nothing forbids you, using current standards, to make a fully AJAX website, sending practically only JSON to your users.

While JSON has a benefit in terms of size (and so bandwidth) and simplicity (slightly easier to parse for smartphones), such major change of replacing one language by another would be too disruptive to justify the minor gains in bandwidth and performance. Such gains were maybe relevant ten years ago where mobile devices were very limited in terms of CPU and when many people had only access to low-speed internet, but today, it's much less relevant.

If you're not convinced, take an ordinary e-commerce website, measure the time spent:

1. Downloading and parsing (sometimes minified and usually gzipped and cached on client side) HTML.

and compare it to the time it takes to:

1. Download minified JavaScript source,
2. Download images,
3. Download CSS,
4. Parse CSS,
5. Render the page,
6. Execute JavaScript.

Now, if you identified that after minifying HTML, gziping it and adding client-side caching, the loading and the parsing of HTML is still the bottleneck on your website, nothing forbids you, using current standards, to make a fully AJAX website, sending practically only JSON to your users.